1. Field of the Invention
The present invention relates to a transmission adaptable to a multi-wheel drive vehicle, such as a four-wheel drive vehicle, e.g., a utility vehicle or an all-terrain vehicle.
2. Related Art
A multi-purpose four-wheel drive vehicle designed to enable off-road traveling, e.g., a utility vehicle or an all-terrain vehicle, repeatedly inclines upward to apply sudden load (peak load) on its rear wheels and inclines downward to apply peak load on its front wheels while traveling on rough ground. In this regard, JP 2000-108695 A (hereinafter, referred to as “Reference 1”) discloses a four-wheel drive vehicle, which is provided with a torque limiter for front wheels, and with another torque limiter for rear wheels, so as to protect its wheel-support system, gears in its traveling power transmission system and its engine from the peak loads applied on the front wheels and the rear wheels. Each of these torque limiters includes input side friction discs and output side friction discs, which are alternately aligned and are pressed against one another by a spring force. When the vehicle travels normally, the output side discs rotate integrally with the input side friction discs so as to transmit power from the engine to the corresponding wheels. When an abnormal torque (peak load) is applied on the front or rear wheels and is transmitted from the wheels to the output side friction discs, the output side friction discs receiving this torque delay to rotate behind the input side friction discs rotated by the engine power. In other words, relative rotation of the output side friction discs to the input side friction discs is allowed so as to prevent the torque applied on the output side friction discs from being transmitted to the input side friction discs. Therefore, the transmission of engine power to the wheels is reduced so as to reduce a burden on a wheel-support system supporting the wheels, and on gears in a transaxle including the wheel-support system. This burden is caused because both the load on the wheels and the engine power resisting the load are applied onto the transaxle and the wheel-support system.
In the vehicle disclosed in Reference 1, a front transaxle incorporates a differential unit carrying axles of the front wheels, and power for driving the front transaxle is taken off from a rear transaxle supporting axles of the rear wheels. This power for driving the front wheels is transmitted to the torque limiter in an input portion of the front transaxle via universal joints and a propeller shaft. Reference 1 discloses only the torque limiter for the front wheels, however, it is obvious for any skilled person that another torque limiter can also be disposed in the rear transaxle carrying the rear wheels.
Further, JP 2010-247671 A (hereinafter, referred to as “Reference 2”) discloses an arrangement of a gear transmission for driving front wheels and rear wheels adaptable to a utility vehicle or an all terrain vehicle. This gear transmission is disposed between a front transaxle for driving front wheels and a rear transaxle for driving rear wheels. Each of the front and rear transaxles includes a housing incorporating a final power transmission mechanism, e.g., a gear train or a differential gear unit, for driving the front or rear wheels. The gear transmission includes another housing that incorporates a transmission gear mechanism. The gear transmission is arranged to continue to a belt transmission driven by an engine so that the transmission gear mechanism is driven by power outputted from the belt transmission. The gear transmission has an output shaft of the transmission gear mechanism. This output shaft is drivingly connected at a rear end thereof to an input portion of the rear transaxle via a rear power transmission device, and is drivingly connected at a front end thereof to an input portion of the front transaxle via a front power transmission device. For example, universal joints and a propeller shaft serve as each of the front and rear power transmission devices.
Further, JP 2004-52789 A (hereinafter, referred to as “Reference 3”) discloses a transmission that is adaptable to a snow blower. This transmission includes a housing incorporating right and left axles and a transmission gear mechanism for driving the right and left axles. This housing also incorporates right and left clutch-type torque limiters for drivingly connecting the respective right and left axles to an output shaft of the transmission gear mechanism.